Who are we to question God's justice? He OWNS the big picture. How many times have I said, "This is unbearable, I'm not going to survive this." Then looked back on it and said, "that was bad, but it turned out to be one the best things that ever happened to me."

Romans 9:14-15

What shall we say then? Is there injustice on God's part? By no means!
For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.”

Bible Gateway link